The present invention relates to a system and method for monitoring brake performance in a vehicle.
Several methods for testing or monitoring brake performance in motor vehicles are known. Such conventional methods are used to indicate to the vehicle operator whether one wheel or individual wheels of the vehicle are being braked too lightly in a defective manner or are otherwise performing poorly (due to glazed or worn brake linings or brake overheating (fading), for example). Without brake performance monitoring, undesirable reduction in overall braking capability would remain largely unnoticed by the operator. Thus, any necessary repair of the defective brakes would be left undone or unnecessarily delayed.
For example, DE 101 39 102 A1 describes a method and device for monitoring the functional capability of a vehicle brake. For this purpose, the difference between the speeds of rotation of the left and right wheels of one axle is evaluated or compared during a braking operation on a straight road. A wheel with higher speed of rotation is considered to be faulty if the difference in speed of rotation exceeds a predetermined threshold. A disadvantage of this method is that the wheel speeds of rotation fluctuate within certain limits due to road influences, such as different coefficients of friction on the left and right, and also cannot be measured with selectively high accuracy. For this reason, the measured result can sometimes be unusable because of external interference.
DE 197 55 112 A1 describes a method and monitoring device for detecting a decline in braking effect of a motor vehicle brake. Brake actuation by the operator or the travel of the brake pedal is compared against the vehicle deceleration achieved thereby. Should the measured vehicle deceleration not correspond to the brake actuation, or in other words be too small, an inadequate effect of the vehicle brakes can be inferred therefrom. A disadvantage of this known method is, in particular, that many other unknown influencing variables, such as engine braking torque, air drag and rolling friction, also act on vehicle deceleration. Furthermore, the underbraked wheel cannot be directly identified. Thus, this method also cannot be regarded as particularly accurate.
U.S. Pat. No. 6,577,942 B1 describes a method for recognizing brake problems based on an evaluation of small variations in wheel slip. The wheel slips of the wheels are measured during a braking operation. The slip values are then compared against slip values from a previous braking operation or current slip values of other wheels. If a relatively large deviation is revealed by this comparison, an alarm is delivered to the operator. A disadvantage of this known method is that accurate measurement of wheel slips also requires that an exact vehicle reference speed be known. Using known means (wheel speed sensors), however, such a speed cannot be determined with the necessary high accuracy during operation of the service brakes, since brake slip causes the wheel speeds in this case to be smaller than the true vehicle speed. This known method is therefore also not excessively accurate.
According to another conventional method described in DE 101 56 607 A1, it is determined by comparison whether a current braking operation is just as effective as a comparable previous braking operation. If the comparison indicates that the current braking operation is less effective, or in other words that the vehicle is being braked less heavily, it is inferred therefrom that the brakes have deteriorated. This known method is suitable, in particular, for temporary, relatively large impairments of the braking effect due to fading (decline of the braking effect due to overheating of the brakes). It also is not overly accurate.
Continuous brake-lining wear of a brake can also be recognized with special wear sensors known to those skilled in the art. However, the use of such sensors represents an undesirable increased cost expenditure.
Also, a wheel braked inadequately by a defective or worn brake can be identified under stationary conditions on a brake test bench.